Method of producing hydrogen peroxide using nanostructured bismuth oxide

ABSTRACT

The method of producing hydrogen peroxide using nanostructured bismuth oxide is an electrochemical process for producing hydrogen peroxide using a cathode formed as oxygen-deficient nanostructured bismuth oxide deposited as a film on the surface of a conducting substrate. An anode and the cathode are immersed in an alkaline solution saturated with oxygen in an electrolytic cell. An electrical potential is established across the cathode and the anode to initiate electrochemical reduction of the oxygen in the alkaline solution to produce hydrogen peroxide by oxygen reduction reaction.

BACKGROUND 1. Field

The disclosure of the present patent application relates to theproduction of hydrogen peroxide, and particularly to a method ofproducing hydrogen peroxide using nanostructured bismuth oxide byelectrochemical reduction of oxygen using an electrode comprising adendritic nanostructured bismuth oxide (Bi₂O_(3-x)).

2. Description of the Related Art

Hydrogen peroxide (H₂O₂) is an essential chemical feedstock for chemicalindustries, medicine and environmental remediation, as well as supplyingan oxidant in renewable energy conversion applications and in storagedevices. Due to its powerful oxidizing nature, H₂O₂ is also used inwater treatment and as an energy carrier in many chemical processeswithout generating toxic by-products. At present, the industrialproduction of high-purity H₂O₂ solution typically relies on ananthraquinone method (i.e., the Riedl-Pfleiderer process), whichinvolves the use of toxic solvents and requires high energy consumption.Transport, handling, and storage of concentrated H₂O₂ produced by themethod raises further safety concerns. Therefore, an effective in situH₂O₂ production technology is desirable.

H₂O₂ may be directly generated electrochemically by oxygen reductionreaction (ORR). ORR in aqueous solutions occurs primarily through twopathways, the direct 4-electron reduction pathway from O₂ to H₂O, andthe 2-electron reduction pathway from O₂ to hydrogen peroxide (H₂O₂).Non-precious metal electrocatalysts with high selectivity for theelectrocatalytic reduction of O₂ to H₂O₂ are desired for theestablishment of green and sustainable chemistry. Bismuth oxide (Bi₂O₃)is a p-type semiconductor material with potential as an efficient ORRelectrocatalyst due to its low conductivity and reactivity. The effectof oxygen vacancies induced in Bi₂O₃, i.e., Bi₂O_(3-x), onelectrochemical generation of H₂O₂ is not known or predicted.

Thus, a method of producing hydrogen peroxide using nanostructuredbismuth oxide solving the aforementioned problems is desired.

SUMMARY

The method of producing hydrogen peroxide using nanostructured bismuthoxide as described herein is an electrochemical approach for producinghydrogen peroxide using a cathode formed as a nanostructured dendritic(ND) oxygen-deficient bismuth oxide (Bi₂O_(3-x)) electrode surface.Bi₂O_(3-x) dendritic nanostructures may be grown on a conductingsubstrate, for example, by first depositing a bismuth film on thesubstrate, annealing the bismuth film in air to convert the bismuth filmto a film of bismuth oxide (Bi₂O₃), and then annealing the bismuth oxidefilm under vacuum to create oxygen vacancies (Bi₂O_(3-x)). Thedeposition step may be electrodeposition. The conducting substrate maybe a transparent conducting substrate, such as fluorine-doped tin oxide(FTO). In use, an anode and the cathode prepared in this manner may beimmersed in an alkaline medium saturated with oxygen in anelectrochemical cell to produce hydrogen peroxide by oxygen reductionreaction.

These and other features of the present subject matter will becomereadily apparent upon further review of the following specification anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a Field Emission Scanning Electron Microscopy (FESEM)micrograph top view of bismuth film electrodeposited on a fluorine-dopedtin oxide (FTO) substrate as a first step in forming an electrode.

FIG. 1B is a FESEM micrograph side view of bismuth electrodeposited onan FTO substrate.

FIG. 1C is a FESEM micrograph of the electrode of FIG. 1A afterannealing the electrode in air to convert the bismuth to Bi₂O₃.

FIG. 1D is an Energy Dispersive Spectrographic (EDS) spectrum of theBi₂O₃ deposited on the electrode of FIG. 1C.

FIG. 1E is a FESEM micrograph of the electrode of FIG. 1C aftersubsequently annealing the electrode under vacuum to form an oxygendeficient Bi₂O_(3-x)/FTO electrode.

FIG. 1F is an EDS spectrum of the oxygen deficient Bi₂O_(3-x) depositedon the electrode of FIG. 1E.

FIG. 2 is a composite X-ray powder diffraction (XRD) diffractogramcomparing patterns for Bi nanostructured dendritic (ND) film, Bi₂O₃ NDand Bi₂O_(3-x) ND on FTO substrates.

FIG. 3 is a composite cyclic voltammetry (CV) voltammogram comparingtraces of Bi₂O₃ ND/FTO electrodes (without being annealed under vacuum)with Bi₂O_(3-x) ND/FTO electrodes (rendered oxygen deficient byannealing under vacuum) in alkaline solution.

FIG. 4 is a composite linear sweep voltammetry (LSV) voltammogramcomparing traces of Bi₂O₃ ND/FTO electrodes, Bi₂O_(3-x) ND/FTOelectrodes, and Pt (20%)/C electrodes in alkaline solution.

FIG. 5A is a composite LSV voltammogram comparing traces taken with aBi₂O₃ ND/FTO electrode in alkaline solution where the solution waspurged for 20 min with either pure nitrogen (N₂), air or pure oxygen(O₂).

FIG. 5B is a composite LSV voltammogram comparing traces taken with aBi₂O_(3-x) ND/FTO electrode in alkaline solution where the solution waspurged for 20 min with either pure nitrogen (N₂), air, or pure oxygen(O₂).

FIG. 5C is a composite LSV voltammogram comparing a trace taken with aBi₂O₃ ND/FTO electrode in alkaline solution saturated with oxygen to atrace taken with a Bi₂O_(3-x) ND/FTO electrode in alkaline solutionsaturated with oxygen.

FIG. 6A is a composite CV voltammogram taken in alkaline solutioncomparing a CV trace of a Bi₂O₃ ND/FTO electrode before addition of H₂O₂with a CV trace of a Bi₂O₃ ND/FTO electrode after addition of 0.4M H₂O₂(30%).

FIG. 6B is a composite CV voltammogram taken in alkaline solutioncomparing a CV trace of a Bi₂O_(3-x) ND/FTO electrode before addition ofH₂O₂ with a CV trace of a Bi₂O_(3-x) ND/FTO electrode after addition of0.4M H₂O₂ (30%).

FIG. 7A is a composite CV voltammogram taken in alkaline solutioncomparing CV traces made upon sequential additions of μM aliquots ofH₂O₂ to the solution.

FIG. 7B is a plot of peak current as a function of H₂O₂content/concentration based upon the traces in FIG. 7A.

FIG. 7C is a composite CV voltammogram taken in alkaline solutioncomparing CV traces made upon sequential additions of mM aliquots ofH₂O₂ to the solution.

FIG. 7D is a plot of peak current as a function of H₂O₂content/concentration based upon the traces in FIG. 7C.

FIG. 8A is a composite LSV voltammogram comparing LSV traces taken atdifferent scan rates for a Bi₂O_(3-x) ND/FTO electrode in oxygensaturated alkaline solution.

FIG. 8B is a plot of peak currents as a function of the square root ofthe scan rate for the traces in FIG. 8A.

Similar reference characters denote corresponding features consistentlythroughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method of producing hydrogen peroxide using nanostructured bismuthoxide is an electrochemical method for producing and sensing hydrogenperoxide using a cathode formed as a nanostructured dendritic (ND)oxygen-deficient bismuth oxide (Bi₂O_(3-x)) electrode. The cathode isformed by depositing a bismuth film on a conducting substrate using anelectrodeposition method, followed by annealing the bismuth film in airto oxidize bismuth to form a film of bismuth oxide (Bi₂O₃), and thenannealing the bismuth oxide film under vacuum to partially reduce thebismuth oxide to form an oxygen-deficient reduced bismuth oxide(Bi₂O_(3-x), where x is greater than 0 and less than 3) surface on theelectrode. The cathode prepared in this manner and an anode are immersedin an alkaline medium saturated with oxygen to form an electrochemicalcell for the production of H₂O₂.

Oxygen deficient nanodendrite Bi₂O₃-x electrodes were controllablyprepared through electrodeposition of bismuth on FTO as an exemplaryconductive substrate, followed by heat treatment in air to oxidize thebismuth and form bismuth oxide (Bi₂O₃), and then by annealing againunder vacuum to create oxygen deficiency and reduce the bismuth oxide.Such electrodes will heretofor be referred to as Bi₂O₃-_(x) ND/FTOelectrodes. The effect of annealing gases on the surface chemistry ofBi₂O₃-_(x) ND/FTO electrodes was examined by cyclic voltammetry (CV) andby scanning electron microscopy (SEM), and compared with Bi₂O₃ ND/FTOand conventional electrodes, with results shown in the drawings. Theovervoltage to perform ORR by cyclic polarization using the exemplaryfabricated Bi₂O₃-x ND/FTO electrodes is considerably reduced relative towhen using the exemplary Bi₂O₃ ND/FTO electrodes. The exemplaryBi₂O_(3-x) ND/FTO electrodes result in efficient production of H₂O₂ atlow overpotential.

The following details the particular materials and methods used in theexemplary implementation of the method. Bismuth (III) nitrate(Bi(NO₃)₃.5H₂O, ≥98.0%) and ethylene glycol (EG; HOCH₂CH₂OH, ≥99.8%)were acquired from Fisher Scientific. All chemicals were used as is.Electrodeposition was carried out in a one compartment cell via a VMP2multichannel potentiostat system. A classical 3-electrode systemcomprising a fluoride-doped tin oxide (FTO) working electrode, anAg/AgCl (4 M KCl) reference electrode, and a Pt counter electrode wasused. Bi-metallic films were prepared starting with a 20 mMBi(NO₃)₃.5H₂O solution in EG. The electrodeposition was performed bypassing 0.1 C/cm² at E=−1.8 V vs. Ag/AgCl, then resting for 2 s. Thecycle was repeated 5 times to pass a total charge of 0.50 C/cm². Theelectrodeposited Bi-metallic films were annealed at 450° C. for 2 h inair after ramping to the target temperature of 450° C. at a ramping rateof 3.0° C./min to form Bi₂O₃ films. The Bi₂O₃ films were placed in aporcelain combustion boat and maintained at 350° C. for various times(0.5 to 5.0 h) under vacuum to obtain Bi₂O_(3-x) films.

The fabricated materials were allowed to cool to room temperature undervacuum. The morphology of the electrodes was examined using FESEM(JSM-6380LA). Ultraviolet-visible diffuse reflectance spectroscopy(UV-DRS) measurements were performed using a Hitachi U-3010. Thecrystallinity and purity of exemplary electrodes fabricated as describedherein were investigated by X-ray diffraction (XRD) on a BrukerD8-Advance Diffractometer via Cu Ka radiation (λ=1.5418 Å).

The theoretical value of the Levich slope (B) is evaluated from thefollowing equation:

B=0:62×n×F×C _(O2) ×D _(O2) ^(2/3) ×v ^(−1/6)

where n is the electron transfer number in ORR, F is the Faradicconstant (96,485 C mol⁻¹), C_(O2) is the saturated oxygen concentrationin 0.1M NaOH aqueous solution (1.2×10⁻⁶ mol cm⁻³), D_(O2) is the oxygendiffusion coefficient (1.73×10⁻⁵ cm² s⁻¹) and v is the kinematicviscosity of the solution (0.01 cm² s⁻¹).

The structure and morphology of the Bi-metallic film, Bi₂O₃ film, andBi₂O_(3-x) film during the electrophoretic deposition and annealingprocesses in the fabrication of Bi₂O_(3-x) ND/FTO were characterized bySEM. FIG. 1A shows FESEM images of the Bi-metallic dendriticnanostructures electrodeposited (charge: 0.5 C cm⁻²) on an FTOsubstrate. The FESEM images show that nano-aggregates of deposited Biform randomly arranged nanodendrites with a micro-nano hierarchicalstructure (see also FIG. 1B) suitable for electrochemical applications.Dendrites in the range of 1-2 μm in length feature many nano-scaleddendrite side branches less than 200 nm in length. Bi ND/FTO electrodesare converted to Bi₂O₃ ND/FTO by annealing in air, as describedpreviously, the resulting nanostructures being shown in FIG. 1C andfurther characterized by EDS analysis, as shown in FIG. 1D. After airannealing, the main branches appear to increase in length and the sidebranches exhibit more defined, leaf-like morphologies. In other words,well-defined nanodendrites are formed in the process of forming theBi₂O₃ film. The FESEM image of a Bi₂O_(3-x) ND/FTO electrode shows nosignificant changes occur in morphology during vacuum annealing, asshown in FIG. 1E. The corresponding EDS analysis is provided in Fig. IF.

The XRD diffractogram patterns of the Bi ND, Bi₂O₃ ND and Bi₂O_(3-x) NDfilms are shown in FIG. 2. The 2θ values may be compared with standardvalues to identify crystalline structures in the material. Thediffraction peaks observed in Bi₂O₃ ND and Bi₂O_(3-x) ND match well withthe standard JCPDS card number 27-0050 of β-Bi₂O₃, which crystallizes ina tetragonal system. The diffraction peaks of the Bi₂O₃ ND andBi₂O_(3-x) ND samples can be indexed well to corresponding singlephases, which crystallize in a tetragonal β-Bi₂O₃ system (JCPDS No.27-0050). The sharp diffraction peaks of Bi₂O₃ ND and Bi₂O_(3-x) NDindicate that each exhibits high crystallinity. However, in the case ofBi₂O_(3-x) ND, the diffraction peaks become much broader and weaker,which indicates its reduced crystallinity relative to the Bi₂O₃ ND.

The electrochemical activity of Bi₂O₃ ND and Bi₂O_(3-x) ND electrodeswas further examined for application as catalysts in ORRs performed inO₂-saturated alkaline solution. For the Bi₂O_(3-x) ND, annealing undervacuum was performed at 350° C. for 120 min. FIG. 3 shows the results ofcyclic voltammograms (CVs) for each of the exemplary electrodesperformed at 50 mV s⁻¹ in 0.1 M NaOH. In the presence of O₂, the Bi₂O₃ND electrode displays a low current plateau in the potential window from0.2 to 1.5 V vs. RHE (reversible hydrogen electrode, used to calibratethe reference electrode). Further, the Bi₂O₃ ND electrode does not showany reduction peak in the measured potential region. In contrast, theBi₂O_(3-x) ND electrode show well-defined reduction peaks at 0.45 V vsRHE. The Bi₂O_(3-x) ND electrode exhibits well-defined high redox peakcurrents, indicating greater electrochemical reversibility than theBi₂O₃ ND electrode. The peak-to-peak separation for the Bi₂O_(3-x) NDelectrode is estimated to be 0.75 V. In comparison to Bi₂O₃ NDelectrodes, oxygen deficient Bi₂O_(3-x) ND electrodes produced by vacuumannealing of Bi₂O₃ ND show considerably enhanced electronic conductivityand reactivity in the ORR process. This could possibly, but withoutbeing bound by theory, be due to the large number of oxygen defectscreated, which provide oxygen vacancies that could serve as acceptors,resulting in semiconducting activity, thereby facilitating reactantadsorption and charge transfer.

During ORR, linear sweep voltammogram (LSV) measurements were carriedout for the Bi₂O₃ ND electrode and the Bi₂O_(3-x) ND electrode, incomparison with state of the art Pt/C catalysts. Results of thesemeasurements are presented in FIG. 4. FIG. 4 illustrates that the oxygendeficient Bi₂O_(3-x) ND electrodes provide significantly moreelectrocatalytic activity compared to Bi₂O₃ ND (but lower than Pt/Ccatalysts in the alkaline electrolyte), demonstrated by the morepositive onset and half-wave potentials. Additionally, the differentelectrocatalytic behavior observed with Bi₂O_(3-x) ND/FTO and Bi₂O₃ ND/FTO electrodes during ORR indicates that more active ORR sites areavailable on the Bi₂O_(3-x) ND/PTO electrodes, possibly due to theavailable oxygen vacancies.

The oxygen reduction potentials are more positive for the Bi₂O_(3-x) NDelectrode relative to the Bi₂O₃ ND and Pt/C electrodes, which suggestsenhanced catalytic performance towards ORR.

To confirm the identity for the oxygen reduction peaks at the Bi₂O₃ NDand Bi₂O_(3-x) ND electrodes, the effect of oxygen concentration inalkaline media was examined by purging the medium with one of O₂, air orN₂. FIGS. 5A-5C show the CV traces taken at exemplary a Bi₂O₃ ND andBi₂O_(3-x) ND electrodes in the different oxygen concentrations in 0.1 MNaOH. The CV for the Bi₂O_(3-x) ND electrodes in deoxygenated (N₂)solution (FIG. 5A) shows no reduction peak in the measured region. TheCV for the Bi₂O_(3-x) ND electrodes in moderate oxygen conditions(electrolyte solution purged with air, FIG. 5B) exhibits oxygenreduction peaks, and in high oxygen conditions (electrolyte purged withO₂, FIG. 5C) shows further increased electrocatalytic current. Finally,these results suggest the observed peaks are due to the reduction ofoxygen at Bi₂O_(3-x) ND electrodes, and we speculate that the reductionprocess at potential 0.46 V vs RHE, which corresponds to the reductionof O₂ by two electrons to give H₂O₂ (or more correctly HO₂ ⁻). Comparingthe CV curves of Bi₂O_(3-x) ND electrodes and Bi₂O₃ ND eledctrodes, itappears that electrocatalytic reduction of oxygen on the electrode ismore effective in the Bi₂O_(3-x) system, which provides greater cathodiccurrent than Bi₂O₃ ND in 0.1 M NaOH aqueous electrolyte. In comparisonwith the Bi₂O₃ ND electrode, the increase of current response ofBi₂O_(3-x) suggests that generated oxygen vacancies play a main part inthe electrochemical production of H₂O₂.

To determine the electrochemical response to H₂O₂, CV was performed inthe absence and presence of 0.4 M H₂O₂, where Bi₂O₃ and Bi₂O_(3-x)electrodes were compared and are demonstrated in FIGS. 6A-6B. FIG. 6Ashows the CVs with addition of 0.4 M H₂O₂ aliquots into 0.1 M NaOHsolution at Bi₂O₃ ND electrode. The Bi₂O₃ ND electrode shows nearly noreduction behavior, demonstrating that the reduction of H₂O₂ was hardlyattained at this electrode. In contrast, the Bi₂O_(3-x) ND electrodedisplays much greater response signals with much greater catalyticcurrent and lowers the over potential value, as shown in FIG. 6B. Thisresult clearly indicates that the oxygen deficient nature of Bi₂O_(3-x)plays a critical role in H₂O₂ reduction behavior.

FIG. 7A displays CVs taken with sequential addition of H₂O₂ aliquots inthe range of 0-40 μM into 0.1 M NaOH solution at Bi₂O_(3-x) NDelectrodes. In N₂ saturated 0.1 M NaOH, the reduction peak current ofH₂O₂ increases gradually following the addition of H₂O₂ concentration.As shown in FIG. 7B, there is a good linear relationship between thepeak current and H₂O₂ concentration in the range of 0˜40 μM (R=0.9957).FIG. 7C displays CVs taken with sequential addition of H₂O₂ aliquots inthe range of 0-100 mM into 0.1 M NaOH solution at Bi₂O_(3-x) electrodes.FIG. 7D displays a good linear relationship between the peak current andH₂O₂ concentration at the range of 0˜100 mM (R=0.984). Upon thecontinued addition of H₂O₂, remarkable current increase at the oxidationpeak is observed, confirming the exceptional oxidizing effect ofBi₂O_(3-x) toward H₂O₂. In addition, the oxidation potential exhibits aslight positive shift, potentially signifying a kinetic limitation ofthe H₂O₂ oxidation reaction.

FIG. 8A displays the influence of the scan rate on the ORR process atBi₂O_(3-x) ND electrodes in 0.1 M NaOH saturated with O₂. Further, theassociation between the cathodic current and the square root of the scanrate is shown in FIG. 8B. For Bi₂O_(3-x) ND electrodes, both oxygenreduction peak currents increase linearly with the square root ofpotential scan rate, signifying that the overall ORR process at thiselectrode is dominated by the diffusion of O₂ from solution to theoxygen vacancies at surface sites. Moreover, with increased H₂O₂concentration, the reduction peak currents shifted toward more negativepotentials, suggesting a possible kinetic limitation in the reactionbetween Bi₂O_(3-x) ND/FTO and H₂O₂.

The Bi₂O_(3-x) ND electrodes allow for production of H₂O₂ at lowoverpotentials. Annealing of metal oxides under vacuum is a simple,scalable and low cost way of creating oxygen vacancies to create highlyefficient catalysts for H₂O₂ generation.

It is to be understood that the method of producing hydrogen peroxideusing nanostructured bismuth oxide are not limited to the specificembodiments described above, but encompasses any and all embodimentswithin the scope of the generic language of the following claims enabledby the embodiments described herein, or otherwise shown in the drawingsor described above in terms sufficient to enable one of ordinary skillin the art to make and use the claimed subject matter.

1-12. (canceled)
 13. An electrode for electrochemical production ofhydrogen peroxide, comprising: a conductive substrate having a surface;and a metal oxide film disposed on the surface of the conductivesubstrate, the metal oxide being partially reduced so that the metaloxide has oxygen vacancies.
 14. The electrode for electrochemicalproduction of hydrogen peroxide according to claim 13, wherein saidconductive substrate comprises fluorine-doped tin oxide.
 15. Theelectrode for electrochemical production of hydrogen peroxide accordingto claim 13, wherein said metal oxide comprises bismuth oxide of formulaBi₂O₃.
 16. The electrode for electrochemical production of hydrogenperoxide according to claim 13, wherein said partially reduced metaloxide comprises oxygen-deficient bismuth oxide of formula Bi₂O₃, whereinx is greater than 0 and less than 3.